Communication system



July 14, 1953 R.A. Hl-:lslNG Ra 23,686

COMMUNICATION SYSTEM Orig. 2 539 623 Original Filed Feb. 12, 1947 A 2 Sheets-Sheet 1 REC.

.4 TTORNE Y .July 14, 1953 R. A. HE'lslNG cowumcmzon sysmn origini Fund Fab. 12, 1947 2 Sheets-Sheet 2 /NVENTOP y R. A. HE/S/NG ATTORNEY Reissued July 14, 1953 COMMUNICATION SYSTEM Raymond A. Helsing, Summit, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Original No. 2,539,623, dated January 30, 1951,

Serial No. 727,973, February 12, 1947.

Application for reissue December 29, 1951, Serial No.

11 Claims.

This invention relates to signaling or communication systems for the transmission of complex wave forms of the type present in speech, music, telegraph, picture and like signals by the process or method sometimes known as pulse code modulation.

` In communication systems utilizing this type of transmission, a speech wave or other signal to be transmitted is sampled periodically to ascertain its instantaneous amplitude. The measured instantaneous amplitude is expressed by pulse codes analogous to telegraph codes.

One code which conveniently may be employed in pulse code transmission involves permutations of a fixed number of code elements each of which may have any one of several conditions or values. An advantageous code of this type is the so-called binary code in which each of the xed number of code elements may have either of two values. One advantageous way of representing these values is to represent one by a pulse sometimes referred to as an on pulse and the other by the absence of a pulse sometimes referred to as an ofi pulse." Alternatively, one value may be represented by a positive pulse and the other by a negative pulse. The total number of permutations obtainable with the binary code is proportional to 2n where n is the number of code elements employed.

Because the total number of diilerent amplitudes which may be represented by such a code of a iixedv number of elements is limited, it is found desirable to divide'the continuous range of amplitude values of which the transmittted signal is capable into a fixed number of constituent ranges which together encompass the total range. Each of these smaller o r constituent amplitude ranges may then be treated as if it were a single amplitude instead of a range and may be [is] represented by an individual one oi' the permutations of the code. In the use of this method of code transmission the instantaneous amplitude ascertained by a sampling operation is represented by the respective permutation indicative of the amplitude range, or step, which most nearly approximates the amplitude of the measured sample. If, for example, the sample amplitude is nearest to that amplitude represented by the ninth step of the signal amplitude range the permutation code corresponding to range 9 is transmitted.

Communication systems based on the use of such codes have in general employed means for sampling the instantaneous amplitude of the complex wave to be transmitted at predetermined intervals, a coder to'express the amplitude at the Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

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time of sampling in the form of a binary code group of a iixed number of code element voltages, means for transmitting the several voltages either simultaneously or successively to a receiver and means for decoding the transmitted code groups to recover the initial complex wave form. Such systems involve careful synchronization of the transmitter and the receiver as well as synchronization between the sampling and coding means which must operate in accurately timed relationship. In many cases the coder operates only intermittently, and in some instances must be reset between samples to prepare it for coding the next sample amplitude.

In other instances it is desirable, through application of well-known multiplex processes, to utilize a single transmission channel for the purpose of transmitting several complex wave forms. It has been usual in such cases to synchronize the transmission of a code group with the operation of the sampling and coding circuits required to produce that code group. 'I'hus it has been necessary to switch a common sampler and coder from one signal input to another, utilizing a single sampler and a single coder to provide representative code groups for each of the signal channels therein. Such operation imposes severe requirements on the sampling and coding equipments which must complete their operation in the time allotted to a single channel.

It is an object of the invention to provide improved and simplified equipment for expressing the instantaneous amplitude of a complex wave form as a binary code group of voltages.

It is a further object of the present invention to provide a code modulation communication system in which operation of the coding means may be substantially continuous and independent of that of the transmitting and receiving equipment.

It is a still further object oi the invention to provide a coder employing feedback to insure accurate representation of the complex wave to be transmitted.

In accordance with the above, the invention relates in one aspect to a communication system comprising a coder adapted for continuously representing the amplitude of a complex wave in binary code form and means for interrupting the operation of the coder momentarily to permit transmission of the code group present at that time to a remote station.

In another aspect the invention relates to a coder comprising a binary counter responsive to the instantaneous amplitude of a complex wave to be transmitted to produce code groups of voltages representative of the amplitude of that complex wave.

In another aspect the invention relates to a communication system for transmitting complex waves in which a code representing the complex wave is generated, the complex wave is regenerated from this code, the regenerated wave is compared with the original wave and flnally the code is corrected in accordance with the comparison.

In other aspects theinvention relates to a reversible binary counter and to the use of such a counter in the above-mentioned coder and to control means responsive to the amplitude of a complex wave to be transmitted for controlling the operation of such a counter.

The above and other objects and features or the invention will be better understood from a consideration of the accompanying detailed description of a specic embodiment taken in conjunction with the drawings in which:

Fig. 1 is a block diagram of a communication system in accordance with the invention; and

Fig. 2 is a circuit schematic of the transmitting equipment shown in block form in Fig. 1.

Referring to Fig. 1 which shows an illustrative embodiment of the invention in block form, a complex wave form to be transmitted, shown here as derived from a microphone I0, is applied to terminal equipment I2. It will be understood that the system of the-invention may be employed with equal benent for the transmission of complex waves from other sources, as for example those from telegraphic, photographic or vibration-actuated devices, and those from television video channels, facsimile transmitters or the like.

Terminal equipment I2 may contain ampliiiers, regulators, impedance transformers, etc. as necessary to convert the output of microphone I or any other source to a suitable power level. The signal appearing at the output of terminal equipment I2 comprising the complex wave form to be transmitted is superimposed upon a direct current bias voltage such that the signal will always be of the same polarity and is applied to apparatus for expressing the instantaneous amplitude of the complex wave form as a binary code group of signals. This apparatus includes a counter I4, an oscillator IB adapted to drive the counter, a comparator IB and an electronic switch arranged to control the operation of counter Il.

Counter Il, which is arranged to measure in arbitrary units the instantaneous amplitude of the complex wave to be transmitted, comprises a multistage binary counting circuit. Each stage of this circuit has two conditions of stability, only one of which may exist at a time. 'Ihe counter is formed by connecting these stages together in such a way that any stage is switched from one o! its conditions of stability to the other only when the next preceding stage has been switched from one condition of stability to the other by one impulse and subsequently switched back to the original condition of stability by a second impulse. In other words two switching operations of any stage are required to produce a single switching operation of the following stage. Thus each stage may be assigned to represent one of the denominational orders or positions of a binary number, the two conditions of stability of the stage corresponding respectively to the zero and the one or that order of the binary number. Then, if means are provided for each stage to produce a signal voltage indicative of the condition of stability occupied by that stage, a binary code representation of the number of impulses applied to the input or rst stage of the counting circuit may be obtained.

Counter I4 is controlled by comparator I8 acting through electronic switch 20. Comparator I8 effectively controls the application of impulses generated in oscillator IG to counter I4 in such fashion that the number of impulses applied is proportional to the instantaneous amplitude of the complex Wave to be transmitted. For this purpose means are included in counter I4 for producing a voltage the amplitude of which is representative of the number of impulses which have been applied to and counted by the counter. This voltage is compared in comparator IB with a voltage proportional to the instantaneous amplitude of a complex wave to be transmitted. If the voltage representing the amplitude of the complex wave is larger than that representing the number of impulses counted by counter I4, electronic switch 20 completes a circuit from oscillator I6 to the counter causing it to operate until the two voltages applied to comparator I8 are equal.

Since the instantaneous amplitude of the complex wave to be transmitted may either increase or decrease, means are incorporated in the counter to permit reversal or the counting direction depending upon whether the voltage representing the complex wave or that representing the output o! the counter is the larger. This reversing means is controlled by another section of electronic switch 20 in response to the output of comparator I8 and conditions counter Il to count upwardly as the instantaneous arnplitude of the complex wave increases and to reverse its counting direction and count downwardly as the intantaneous amplitude of a complex wave decreases.

As pointed out above, counter I4 provides an individual output signal for each stage and each output signal may have either of two values indicating which of the two conditions of stability is occupied by the stage of the counter from which it is derived. The several output signals taken together form a binary code representation of the number of impulses counted and thus of the instantaneous amplitude of the complex wave applied through terminal equipment I2. These output signals are applied individually to a radio transmitter 22 capable of producing a different subchannel carrier frequency for each of the counter output signals, one carrier being controlled by each of the counter output voltages. At predetermined instants, xed in the case of this illustrative embodiment by timing oscillator 24, a gating or timing system 26 interrupts the action of the coding system and renders the transmitter operative to radiate simultaneously all subchannel carriers which have been enabled.

At the receiving end o1' the system, the radiated signals areA received by radio receiver 28 and the demodulated output of that receiver is applied to a decoder 30. This decoder operates in response to the transmitted binary code groups to reconstruct the complex wave form represented thereby and its output is applied to terminal equipment 32 which comprises the necessary amplifiers, iilters and impedance changing devices required to produce an output suitable for application to headphones 34 or any other output device which it may be desired to employ.

Circuit details of illustrative transmitting equipment in accordance with the invention are given in Fig. 2 to which reference is now made. The complex wave signal appearing at the output of terminal equipment I2 in Fig. 1 is applied through terminal 3E and potentiometer 50 to a comparator indicated at I8 and comprising vacuum tubes 38 and 40 together with plate and bias batteries 42 and 44, respectively, and load resistors 46 and 48. Potentiometer 5l!` is connected to ground through a bias battery 5I by means of which a direct current bias voltage is added to the complex Wave signal at the output of the terminal equipment. This bias voltage has an amplitude which is at least equal to one-half the maximum peak-to-peak amplitude to be accepted by the coding circuits. Thus all signals applied to the coding circuits Will be oi one polarity. Assuming for example and as shown that the bias voltage is positive, and is equal to onehalf the peak-to-peak signal accepted by the coding circuits, then the maximum signal amplitude applied to the grid of comparator tube 38 will be equal-to the full amplitude which can be accommodated whenever the complex wave signal reaches its peak positive value. On the other hand, the signal applied to tube 38 of the comparator will be substantially zero when the complex Wave signal reaches its peak negative amplitude. Accordingly, tube 38 is never cut oi and will operate at the approximate center of its range when no complex wave signal is applied (or when the complex Wave amplitude is zero). 'Ihe signicance of this condition will appear below.

The signal voltage appearing across potentiometer 50 is applied to the control grid of vacuum tube 38 as stated above while a potential derived from counter I4 and appearing across resistor 52 is applied to the control grid of vacuum tube 40. The potential applied to the comparator from the counter is derived therefrom in a manner which will be described hereinafter and is proportional in amplitude to that represented by the binary code group appearing at the output of the counter. Comparator I8 controls the operation of counter I4 through electronic switching circuits to determine when and in which direction the counter shall operate.

Counter I4 comprises an impulse counting circuit and certain auxiliary circuits permitting reversal of the counting direction thereof. The counting circuit comprises a plurality of stages each of which may include a switching circuit of the type sometimes referred to as a flip-flop circuit and having two mutually exclusive conditions of stability. Three such counting stages 54, 56 and 58 are shown in Fig. 2 representing the first, second and nth stages of a n-stage counter. The dotted connections in Fig. 2 indicate the location of additional intermediate stages which are identical to stage 58 as shown. It should be understood that the number of counting stages employed depends in any particular instance, upon the range of amplitude desired to be accommodated by the transmission system and upon the required fidelity of transmission. If, for example, three stages are employed each having two conditions of stability, the possible number of unique permutations among the operating conditions obtainable by switching the three counting stages individually from one condition of stability to another amounts to 23 or 8. Similarly the use of iive counting stages results in an increase in the number of possible permutations to 25 or 32. The nstage counter forming a portion of the coder of Fig. 2 accordingly permits expression of 2n permutation values.

Each of the counting stages 54, 56 and 58 comprises a pair of vacuum tubes 60 and 62, 84 and G6, and 68 and l0 respectively. Considering the iirst of these identical counting stages, impulses appearing at terminal 'I2 are applied through capacitors 14 and 16 to the control grids of vacuum tubes G0 and 62, respectively. The cathodes of tubes S0 and 62 are connected together and to ground while the plates thereof are connected respectively through inductor 'I8 and load resistor 8U in series and inductor 82 and load resistor 84 in series to the positive terminal of battery 86, the negative terminal of which is grounded. The plate of tube 6!) is connected through tapped resistor 88 to the negative terminal of bias battery 98, the positive terminal of which is grounded while the plate of tube 82 is connected through tapped resistor 92 to the negative terminal of bias battery 90. The control grids of tubes 6U and E2 are connected respectively to the taps of resistors 92 and 88. Thus a portion of the potential appearing at the plate of tube E0 is applied to the control grid of tube 62 and similarly a portion of the potential appearing at the plate of tube 82 is applied to the control grid of tube 68.

The circuit arrangement including vacuum tubes 60 and 62 and the associated circuit components and sources of potential is stable with current flowing in either one but not both of the tubes. Thus if it be assumed initially that current is ilowing through tube 82, tube SII is cut on and the operation of the circuit may be considered as follows. When current is flowing through tube 62 the plate potential thereof will be lower than that of tube because of potential drop in resistor 84 while no potential appears across resistor 80 as there is no current in tube 60. Since the grid of tube SII has a connection to the plate of tube 62 through resistor 92, this grid is maintained suificiently negative to prevent current flow through tube 60. Since no current is nowing in tube G0 the plate potential thereof is high and this potential applied to the grid of tube 62 through resistor 88 maintains the potential of the grid of tube 62 at a suitable point to permit current flow through that tube. If now a negative impulse is applied through terminal l2 and capacitors 14 and I8 to the control grids of vacuum tubes 60 and 62 the following action occurs. Since tube 60 is already cut off the negative impulse applied to the grid thereof has no direct effect. On the other hand the negative impulse applied to the control grid of tube 82 tends to cut on the plate current in that tube and correspondingly increase the plate .potential thereof. The diminution of current in tube 62 causes the potential drop across resistor 84 to disappear and allow the potential at the plate of tube 82 to approach that of the plate of tube E8 and in addition the falling current in tube 62 causes inductance coil 82 in the plate circuit to produce an additional potential change, acting to raise the potential of the plate of tube 82 well above that of the plate of tube 60. Then due to the connection between the plate of tube 82 and the grid of tube 80, the potential of the grid of tube 60 is increased suiciently to start current owing in this tube. As the plate current begins to flow in tube 60, a reduction of its plate potential results, reducing further the voltage on the grid of tube 62 to maintain that tube cut off. The current flow is thus switched from tube 62 to tube 60 and the circuit is shifted from one of its conditions of stability in which tube 62 conducts while tube 60 is cut off to the other in which the reverse situation obtains.

If a second negative impulse is applied through capacitors 14 and 16 to the control grids of the two tubes a similar process occurs. Plate current through tube 60 is cut off. The energy stored in inductor 16 is sufficient to cause initiation of plate current flow in tube 62 returning the nip-flop circuit to the initially assumed condition of stability. Thus it will be seen that for each pair of negative impulses applied to counting stage 64 the ip-op circuit shifts from one condition of stability to the other and then shifts back to its original condition completing a counting cycle.

For .purposes of discussion, it is convenient to designate the two conditions of stability of such flip-flop circuits as the off and on states, calling the tube which is conducting in the first of these states the off tube and that which is conducting in the second the on tube. If it is assumed that when current ows in tube 62, counter stage 54 is in the olf state, tube 62 is the off tube and tube 60 is the on" tube. This convention will be followed hereinafter in referring to the tubes of the counter circuit.

The circuit arrangement of counting stage 56 is identical to that of counting stage 54, positive potential from battery 66 being applied to the plates of vacuum tubes 64 and 6I through load circuits including respectively inductor 64, resistor 96 and inductor 60, resistor |00. Likewise the plate of tube 66 is connected through tapped resistor |02 to the negative terminal of bias battery 60 and the plate of tube 64? is connected through tapped resistor |04 to the same point. The grids of vacuum tubes 64 and 66 respectively are connected to the taps on resistors |02 and |04 and impulses may be applied to these gridsthrough capacitors I1 and I I6 which correspond to capacitors 14 and 16 of counting stage 54. As in the case of the counting stage described above, counting stage 56 has two conditions of stability and the current now is shifted from one of the tubes to the other by the application of a negative impulse through capacitors ||1 and I6 to the grids of the two tubes. As in counting stage 54 two negative impulses must be applied successively to the grids to cause one complete counting cycle. Conforming to the convention chosen above, tube 66 of the counting stage 56 will be designated the off tube and tube 64 the on tube.

The interstage connections which control the direction of counting will now be considered. In Fig. 2 the plate of on tube 60 of counter stage 54 '1s connected through capacitor |06 to the control grid of vacuum tube |06 while the plate of off tube 62 is similarly connected through capacitor to the control grid of vacuum tube ||2. The plates of vacuum tubes |06 and ||2 are connected together and through resistor ||4 to the positive terminal of battery I I6, the negative terminal of which is grounded. The grid of tube |06 is connected through series resistor ||6 to the junction of voltage divider resistors |20 and |22 which are connected in series between the negative terminal of battery |24 and control lead |26. Similarly the control grid of tube ||2 is connected through resistor |28 to the junction of resistors |30 and |32 connected in series between the negative terminal of battery |24 and control lead |34.

The negative voltage from bias battery |24 is made suflicient to maintain both interstage tubes |06 and I|2 cut off in the presence of the largest possible potential difference between control leads |26 and |34. If, however, one of the control leads, |26 for example, is at a higher potential than the other control lead |34, tube |08 will be biased just below cut-off by the joint action of the bias voltage from battery |24 and that on control lead |26 while tube ||2 will be driven far below cut-off.

Let it be assumed that control lead |26 is positive with respect to control lead |34 and that interstage tube |08 is operating just below cut-off with tube I2 far beyond cut-oil and that counting stages 54 and 56 are in the oil condition with off tubes 62 and 66 conducting and on" tubes 60 and 64 cut 01T. If a nega-tive impulse is applied through termina1 12 and capacitors 14 and 16 respectively to the control grids of tubes 60 and 62, current flow will shift from oif tube 62 to "on" tube 60 as has been explained above. Thus the plate voltage of oif" tube 62 increases while that of on tube 60 decreases.

Since interstage tube |I2 is biased far below cut-off, the increase of voltage at the plate of off tube 62 has no effect on the operation of tube ||2. Similarly, the drop in plate voltage of on tube occasioned by the initiation of current fiow therethrough has no effect on the operation of tube |08 which is already out off as stated above.

If a second impulse is now applied to the grids of vacuum tubes 60 and 62 of counting stage 54, this stage is returned to its initially assumed off" condition with current lo'w shifted back from on tube 60 to off" tube 62. As off tube 62 again becomes conductive its plate voltage drops but this voltage drop, applied to the grid of interstage tube ||2 has no effect because this tube is already biased far beyond cut-off.

On the other hand, the increase in plate voltage brought about by the return of on tube 60 to the cut-off condition and applied through capacitor |06 to interstage tube |06 is suicient to cause the flow of plate current through this tube which, it will be recalled, was assumed to be biased just below cut-off by the potential across voltage divider resistor |20. The increase in plate current in tube |06 causes a corresponding drop in plate voltage and thus in the voltage applied through capacitors I1 and I9 to the grids of vacuum tubes 64 and 66, respectively, of counter stage 56. This drop in plate voltage corresponds to a negative pulse and is eifective to shift the flow of current in counter stage 5S from 01T tube 66 to on tube 64.

Thus it will be seen that two negative impulses must be applied at terminal 12 to produce one shift in current how in counter stage 56 and that by the same analysis four negative impulses must be applied to terminal 12 to produce a full counting cycle of counting stage 56, shifting the flow of current from the oil tube 66 to the on tube 64 upon the second impulse and back to the off tube 66 upon the fourth count.

As shown in Fig. 2 the plates of off tubes 62, 66 and 10 of the first, second and nth counter stages 54, 56 and 58 are connected respectively to counter output terminals |36, |38 and |40. In the operation of the counter in the manner described above, it is seen that upon the application of va single negative impulse under the initial conditions assumed above with current flowingr in off tubes 62, 66 and 10, the voltage at terminal |36 is increased to a value greater than that initially present. Thus increased output voltage is present at terminal |36 while that at terminal |38 remains at the initial value. Thus the voltages on terminals |38 and |36 in that order may represent the binary numl ber 01, corresponding to a count of one impulse.

Upon receipt of a second negative impulse counter stage 54 returns to its initial condition completing a full cycle with the current ilow again through "03 tube 62, and applying a single negative impulse to counter stage 56 to shift the flow of current therein from ofl" tube 66 to on tube 64. Thus the voltage at terminal |36 is reduced and an increase in output voltage appears at terminal |38. In this instance the binary number obtained on terminals |38, |36 in that order is 10 corresponding to a count of two.

A third impulse applied at terminal l2 again switches counter stage 54 to the on condition (as contrasted with the initially occupied oi" condition) and the flow of current is again switched from off" tube 62 to "on" tube 60. Accordingly, the plate voltage of off tube 62 increases and the voltage appearing at output terminal |36 is increased signifying operation of the stage. Since the third impulse causes only one-half cycle of operation of counter stage 54 no negative impulse is applied to counter stage 56 and it remains in the on" condition to which it was switched by the second impulse. Thus after three impulses signal voltages appear at both terminals |38 and |36 indicating a binary number 11 corresponding to the number 3.

Additional stages of the counter, as for example a third counter stage inserted with interstage tubes at the location of the dashed lines of Fig. 2, operate in an entirely analogous manner. Thus, if the current in the third stage is initially through the o' tube, the third counter stage remains in its initial off condition in which the current iiow is through the 0|" tube. Upon the -application of a fourth impulse at terminal l2, however, counter stage 54 returns to the unoperated condition removing the output voltage from terminal |36 and at the same time applying a negative impulse to counter stage 56. This impulse serves to switch counter stage 56 from the on condition of stability which it occupies at the conclusion of the third impulse to the off condition removing the signal potential from terminal |38 and at the same time applying a negative impulse to the third counter stage. This negative impulse switches the ow of current from the oflm tube of this stage to the on tube and the increase in [late] plate potential of the off tube produced by this action may appear at an output terminal connected to the plate of the oiT tube as an output signal indicative of the operation of this stage.

'Ihe operation of three counter stages in counting four impulses may be summarized in a table as below:

Let it now be assumed that after the counter has reached a count of three with current flow in on" tubes 60 and 64 and increased output voltages appearing at both terminals |38 and |36. the potential between control leads |26 and |34 is reversed in polarity, as would be the case if the complex wave amplitude were to decrease. As a result of such reversal of control potential, interstage tube |08 is carried well below cut-off while the bias on interstage tube ||2 is increased until that tube is just below cut-off. If such adjustments in the operation of the interstage tubes are made by means which will be considered hereinafter, the counter comprising counting stages 54, 56 and 58 may be caused to reverse and count downwardly in response to the application of further negative impulses at terminal l2. If, for example, after the counter has reached an indication of the decimal value 3 as described above and the interstage connections have been readjusted as just assumed, a negative impulse is applied at terminal l2, the following action occurs.

'Ihe negative impulse applied at terminal l2 switches the current flow in counter stage 54 from on tube 60 to olf tube 62 placing this stage in its olT' condition with the result that an increase in potential appears at output terminal |36. This switch in current flow results in an increase in the plate potential of on" tube 6|] and a decrease in plate potential of 05" tube 62. Since interstage tube |08 is far below cutoif, the increase of plate voltage of on tube 60 will be of no effect. Also, since tube ||2 is still cut off though now operating just below cut-off, the decrease of plate voltage of ofl tube 62 will not cause any current flow in tube I l2. Consequently, the potential at output terminal |38 will remain high while that at terminal |36 is reduced to its lower value and the binary number on these terminals will be 10, correspon-ding to the count of two.

Upon application of another negative impulse at terminal 12 (the second impulse after reversal of the control potential), current flow in counter stage 54 is again switched, it being transferred from 01T tube 62 to on tube 60. The resultant decrease in the plate voltage of on tube 52 has no further effect since interstage tube |08 is cut oil. The increase in plate voltage of off tube 62, however, is sullicient to cause current flow in interstage tube ||2 with a resultant drop in the plate voltage thereof. Accordingly, a negative impulse is applied through capacitors Il'l and Il! to the grids of tubes 64 and 66 in counter stage 56. Thus current ilow for counter stage 56, which was in on tube 64 at the counts of two and three, is shifted to off" tube 66 with a resultant decrease in output potential at terminal |38. The binary number on output terminals |38 and |36 is then made 0l correspond ing to a count of one.

From the above it will be seen that the direction of counting may be reversed and the counter caused to count downwardly by reversing the potential on control leads |26 and |34. While the above consideration has been confined to the two counting stages 54 and 56, it will be apparent that a counter with any desired number of stages may be caused to operate in the same fashion through employment of interstage circuits similar to that shown between stages 54 and 56, between each pair of stages.

In the above it has been assumed that means were available for turning the counter on and oil' and for controlling the direction of counting by adjusting the potential between control leads |26 and |34. Comparator |8 performs these functions by comparing a voltage proportional to the instantaneous amplitude of the complex wave (plus the bias from battery applied to input terminal 36 (Fig. 2) with a potential which has been defined above as being proportional to the number of impulses indicated as counted by the binary number appearing on the output terminals of the counter. This potential is derived from the counter in the following manner. Plate-grid resistors 92, |02 and |42 of counter stages 54, 56 and 58 respectively are tapped and the taps are connected through rectiflers |44, |46 and |46 respectively and series resistors |50, |52 and |54 respectively to the grid of vacuum tube 40 which forms a part of comparator I8.

The locations of the taps on resistors 92, |02 and |42 are so chosen that when the respective counter stages are in the off" condition with current ilowing in the off tubes, the potentials derived therefrom are slightly below ground potential, the negative potential furnished by bias battery 90 being suillcient to overcome the relatively low positive potentials at the plates of the off tubes during the time these tubes are conducting. Under these conditions rectiilers |44, |46 and |48 effectively prevent any potential from appearing across resistor 52 and being applied to the control grid of triode 40.

II for example, counting stage 54 is switched to the "on condition, the plate potential of ou tube 62 and consequently the voltage at the tap of resistor 92 will rise and current will ow through rectifier |44, resistor |50 and resistor 52. Resistor |50 is so proportioned that the drop across resistor 52 in response to such current iiow is proportional to the amplitude portion represented by operation of counting stage 54. Thus by way of example, resistor |50 may have such an ohmic value that 0.1 volt is developed across resistor 52 when counting stage 54 is switched to its on condition denoting a count of one.

The circuits including rectier |46-resistor |52 and rectifier |48-resistor |54 operate in the same manner when the stages with which they are associated are switched to the on condition of stability. In the case of stage 56, however, resistor |52 is so proportioned that the iiow of current through resistor 52 occasioned by operation of counting stage 56 is proporti-onal to the amplitude increment of 2 represented by operation of that stage. If the potential drop across resistor 52 in response to the operation of counter stage 54 is made 0.1 volt as suggested -by way of example above, the drop across this resistor produced by operation of counter stage 56 must then be 0.2 volt. Similarly, the drop across resistor 52 produced by operation of the nth counting stage 58 must be proportional to the increment of amplitude represented by operation of that stage or [D.1 2] 0.1 :1: 2"-1 volts. Voltages developed across resistor 52 upon the operation of additional stages must, of course, ybe proportional in the same way to the amplitudes represented by operation of the stages. It will be understood that the voltages developed across resistor 52 by the circuits considered above need not be respectively 0.1, 0.2 [0.1X2n] 0.1 :n 2"-l volts, but may have any values in the same proportion so long as the current flowing in resistor 5| in response to operation of the counting stage representing the largest amplitude increment does not disturb the current owing in that resistor from the counting stage representing the smallest increment beyond an amount which would interfere with the operation of the system with the fidelity required for a particular application.

From the above it will appear that the voltage developed across resistor 52 and applied to the control grid of triode 40 is proportional to the amplitude of the counting sum appearing as a binary number on counter output terminals |36, |38 |40.

It will be recalled that battery 5| in series with potentiometer 50 in the grid circuit of comparator tube 36 is eiective to add a steady bias voltage to the complex wave applied at terminal 36. Conveniently the -bias voltage so added is positive and equal to one-half the peak-to-peak amplitude of the largest complex wave to be accepted by the coder. Thus all complex wave amplitudes applied to the grid o ff comparator tu-be 38 will be positive and the signal applied thereto for zero complex wave input at terminal 36 will be substantially one-half the total range acceptable by the coder.

If, for example, the maximum peak-to-peak amplitude is A, the bias voltage from battery 5| may be a which is equal to 1/2 A. Then, measured [in] with respect to ground. the complex wave as applied to the grid of comparator tube 38 will have a range of values extending from (ll to 2a and all such values will be positive [in] with respect to ground. If potentiometer 50 in the grid circuit of comparator tube 38 is so adjusted that the voltage applied to the grid of this tube for a given instantaneous complex wave amplitude, measured with the superimposed bias, is equal to that developed across resistor 52 when the counting sum has an amplitude equal to the instantaneous amplitude of the complex wave in arbitrary units, it will be seen that the instantaneous amplitude of the complex wave may be continuously compared with the counting sum appearing on terminals |36 through |40 of the impulse counter. Such comparison is made .by tubes 3l and 40. the plate potentials of which vary respectively and inversely as the complex wave amplitude applied through potentiometer 50 and measured as set forth above and the counting sum amplitude applied through resistor 52. Thus if the amplitude of the complex wave at any instant exceeds that indicated by the counter, the plate potential of tube 38 will be less than that of tube 40 and a potential difference will exist between leads |56 and |56 connected respectively to the taps of resistors |51 and |58 which are connected between the plates of tubes 38 and 40. respectively and the negative terminal of bias battery |24. Similarly a potential difierence of the opposite polarity will occur when the amplitude of the complex wave is less than that of the counting sum output of the counter. These differences in potentia1 are employed to turn the counter on whenever the two values are unequal and to determine the direction in which the counter is to operate or count.

In the foregoing it has been assumed that negative impulses have been applied to terminal 12 of the counting circuit without reference to the source of these impulses. In the illustrative system of Fig. 2 such impulses are derived from an oscillator |60, the frequency of which must be sumcient to permit operation of the counter over its entire counting range with enough speed to permit accurate representation of all frequency components o! the complex wave form desired to be transmitted. For example, oscillator |60 which may be of any conventional type may have a frequency of 500,000 or 600,000 cycles per second. Its output is coupled through transformer |62 to a pair of control tubes |64 and |88 shown herein as tetrodes. The secondary winding of the transformer is center tapped and the push-pull signal developed in this winding is applied to the screen grids of the two control tubes. The cathodes of these two tubes are connected together and to ground while the plates are connected through a common load resistor |68 to the positive terminal of battery |10, the negative terminal of which is grounded, Resistors |51 and |58 connected to the plates of comparator tubes 38 and 40. respectively are tapped to provide suitable potentials for application to the control grids of counter control tubes |64 and |66.

If the amplitude of the counter sum output is equal to the instantaneous amplitude of the complex wave to be transmitted, as measured from ground after the addition of the bias voltage from battery no potential difference appears between comparator output leads |56 and |58 and the control grids of tubes |64 and |66 are held at equal potentials. Accordingly, the oscillator signal applied in push-pull to the screen grids of these tubes is canceled out in the plate circuit and no alternating current output is produced. If on the other hand, a potential diierence of either polarity occurs between the output leads from comparator tubes 38 and 40 in response to a disparity between the instantaneous amplitude of the complex wave to be transmitted, measured as above, and the amplitude of the sum output of the counter. the current through one of control tubes |64 and |66 will exceed that through the other and an alternating current component will appear across plate resistor |68.

The potential developed across plate resistor |68 of counter control tubes |64 and |66 is applied to the screen grid of a switching or gating tube |12 having at least two grids. The cathode of this tube, which is shown as a tetrode. is grounded and the plate is connected through load resistor |14 to the positive terminal of battery |16, the negative terminal of which is grounded. It will be assumed for the purpose of the present consideration that the potential applied to the control grid of switching tube |12 is such as to permit conduction of current therethrough whenever the voltages applied to the plate and screen grid have suitable values. It will be noted that the screen grid of switching tube |12 is connected directly to the plates of counter control tubes |64 and 66. Accordingly, a positive voltage of varying amplitude is appplied to the screen grid of switching tube |12. This voltage is so chosen in relation to the other voltages applied to tube |12 (by variation of the potential of battery |10 or load resistor |68) that this tube is rendered conductive only when the sinusoidal plate signal across resistor |68 reaches a positive peak and the voltage on its control grid is simultaneously great enough to permit the flow of current. Thus, if the voltage applied to the control grid of switching tube |12 is suitable and if, because the sum output of the counter is not representative of the instantaneous amplitude of the complex wave to be transmitted, a sinusoidal voltage appears across load resistor |68, switching tube |12 will pass plate current for each positive peak of the applied sinusoidal voltage. Accordingly,

a series of negative pulses are developed across resistor |14 whenever the counter output is not representative of the complex wave amplitude. I'hese negative pulses are applied at terminal 12 to the first stage of the counter, resistors 88 and 82 and capacitors 14 and 16, respectively being proportioned to differentiate and thus sharpen the negative pulses applied to counting tubes 60 and 62.

The circuits thus far described illustrate the manner in which the counter is turned on or oil depending upon whether or not the sum indicated thereby is representative of the amplitude of the complex wave to be transmitted. Means for controlling the direction of counting of the counting circuit will now be considered. For this purpose the potential difference existing between comparator output leads |56 and |58 is ampliiled and applied to control leads |26 and |34 to control interstage tubes |08, ||2, etc In Fig. 2 the amplifler is shown as comprising vacuum tubes |18 and but a multistage amplifier may be employed, the principal requirement being that when the counter sum differs from the complex wave amplitude by an amount equal to a count of one unit, the voltage developed between control leads |26 and |34 must be suiicient to apply the proper bias to the interstage tubes as considered above. The cathodes of tubes |18 and |80 of the amplifier 'shown in Fig. 2 are connected together and to ground while the plates thereof are connected through load resistors |82 and |84 respectively to the positive terminal of tapped battery |86, the negative terminal of which is connected to ground. The screen grids of these tubes |18, |80 are connected together and to the tap of battery |86 while the control grids are connected respectively to the taps on comparator output resistors |58 and |51. In Fig. 2 the grids of amplier tubes |18 and |80 have been shown as connected to the same taps on resistors |58 and |51 as the grids of control tubes |66 and |64. It will be understood, of course, that these two sets of connections need not be made to the same taps and may be varied as desired to obtain the proper potentials for application to the grids of the control and amplii'ying tubes.

In considering the operation of the counter directional control circuit, let it be assumed that the counting sum voltage appearing across resistor 52 is less than the instantaneous amplitude of the complex wave to be transmitted, measured with respect to ground and applied to the grid of comparator tube 38. Under these conditions the plate current in tube 38 exceeds that in tube 40 and as a result, the plate voltage of tube 38 drops below that of tube 40. Accordingly, the voltage applied to the grid of amplifier tube |80 is less than that applied to amplifier tube |18. As a result, the plate current in amplifier tube |80 vis less than that in amplifier tube 18 and the plate potential of amplier tube |80, and thus'the potential on control lead |26. is higher than the plate potential of amplifier tube |18 and thus the potential on control lead |34. Then the interstage tubes |08, 86, etc. are biased just below cut-oil' and can pass positive-pulses from the on counter tubes, while interstage tubes |2, |88, etc. are biased far below cut-off and eiectively open the interstage circuits between the 01T tubes of the counter. Thus the counter will count upwardly as has been described above, tending to increase the counting sum and thus to balance the comparator control circuit.

In a similar fashion, the counting. direction of the counter is reversed whenever the sum output Voltage appearing across resistor 52 exceeds the complex wave amplitude as measured from ground after imposition of the bias from battery 5|. Because of the bias from battery 5| the counter will rest at the approximate middle of its range whenever the complex wave applied at terminal 36 is of zero amplitude. As the complex wave at this point becomes more positive, the counter will count upwardly and as the applied complex wave becomes negative (at terminal 38) the counter will reverse and count downwardly.

From a consideration of the circuit arrangements described above, it will be seen that the potentials appearing at counter terminals |40 |30 and |36, taken in the order given, form a binary code group representative of the instantaneous amplitude of the complex wave applied at terminal 36 from terminal equipment I2. This binary code group changes continuously so that it is maintained at all times representative of the instantaneous amplitude of the complex wave as that amplitude increases and decreases.

The potentials at terminals |40 |38 and |38 may be transmitted to a remote station by a variety of means. For example, individual transmission subchannels each comprising a separate wire transmission line might be provided or known methods of multiplex may be employed. Conveniently and as shown in the embodiment illustrated in Fig. 2 means are provided for transmitting the code group formed by these output potentials over a radio transmission link comprising a separate subchannel for each of the counter stage output voltages. Because of the requirements of the particular receiver with which the exemplary system described herein is designed to operate, means are provided to render this radio transmission link operative only at predetermined times, during which times the coder is momentarily stopped to permit transmission of a fixed binary code group to the receiving station. In other cases it may be desirable, depending upon the receiver employed, to operate both the coder and transmitter continuously or to operate the receiver periodically interrupting the coder during the transmitting intervals as in the exemplary system.

'Ihe radio transmitter of the exemplary system comprises power amplifier tubes |90, |92 and |94, which may be of the pentode type, associated respectively with counter stages 54, 58 and 58. The cathodes of power amplifier tubes |90, |92 and |94 are connected together and to ground while their plates are respectively connected through transformers |98, |98 and 200 to a. dipole antenna 202, plate potential for the tubes being applied from battery 204 through the primary windings of these transformers. Associated with the power amplifier tubes |90, |92 and |94 are radio frequency oscillators 206, 208 and 2| 0, respectively. If additional counting stages are employed, additional amplifiers and oscillators must be provided, one oscillator and one amplifier being associated with each counting stage as shown in Fig. 2. The oscillators are tuned to different radio frequencies and their outputs are applied to the suppressor grids of the associated amplifier tubes superimposed upon a bias voltage derived from battery 2|2. The counter output voltages from counter terminals |36, |38 and |40 are applied respectively to the screen grids of amplifier tubes |90, |92 and |94. If a suitable potential is applied to the control grids of amplifier tubes |30, |32 and |34, the radio frequency signals generated by the oscillators, respectively associated with these amplifier tubes will be applied to the antenna whenever the counter stage associated with a particular amplifier tube is in its "on condition. When a counter stage is in its on condition, the potential appearing at its output and applied to the screen grid of the associated amplifier tube is insufficient to permit conduction.

The radio transmitter is turned on and the operation of the counter interrupted momentarily by means of a timing circuit to permit the transmission of a code group representation of the instantaneous amplitude of the complex wave. In the exemplary embodiment described and illustrated herein the transmitting process is made to occur periodically under the control of an oscillator 2|2 which may be any conventional oscillator capable of producing a sinusoidal output signal of relatively constant frequency. If aperiodic transmission is desired a device capable of producing impulses at random intervals may be substituted for the oscillator. One such device may include means for amplifying a random noise potential and clipping it to produce a series of irregularly spaced impulses suitable for the operation of the timer.

In the present embodiment in which transmissions occur at regular intervals oscillator 2|2 may conveniently have an operating frequency of approximately 8,000 cycles per second. At this frequency at least two code groups will be transmitted for all frequency components of the complex wave to be transmitted having frequencies equal to or less than 4,000 cycles per second. If it is desired to transmit higher frequency components the oscillator frequency must necessarily be increased, and this may be done without substantial limitation so long as the counter is capable of suniciently rapid operation to permit counting of the full amplitude range between the times of transmission. Since the counter operates reversibly and continuously measures the instantaneous amplitude of a complex wave to be transmitted, it is not necessary to allow any time for resetting the counter. Accordingly, for a given counter speed, higher frequency components may be transmitted over the system of the invention than over those previously known.

Returning now to the illustrative embodiment shown in Fig. 2, the sine wave output of oscillator 2|2 is supplied through series resistor 2 I4 to the control grid of a vacuum tube 2 I 6. The cathode of this tube, which may conveniently but not necessarily be a triode, is grounded while the plate is connected through load resistor 2|8 to the positive terminal of battery 220, the negative terminal of which is connected to ground. Series resistor 2|4 in the grid circuit of this tube tends to flatten the sine wave and produces a signal across load resistor 2|8 having a wave form of the type shown at 222. This signal is applied to a circuit comprising resistor 226 and capacitor 224 which are of such values that the signal is dierentiated, producing a wave form of the type shown at 228. The positive portions of this differentiated signal are eliminated by rectier 230 through which the signal is impressed upon resistor 232 and capacitor 234 connected in parallel. The negative portions produce charges on capacitor 234 that are discharged relatively slowly through resistor 232. The resultant negative saw-tooth pulses 238 are impressed through bias battery 238 upon the con- Bias battery trol grid 0f Switching tube |12.

238 is of a value to permit current flow through switching tube |12 and each of the negative peaks of the signal at 236 serves to cut on the flow of current through this tube |12 to interrupt the operation of the counter. The values of capacitor 234 and resistor 232 are so chosen that switching tube |12 is cut olf for only a small part of the operating cycle of the timer.

The voltage appearing across resistor 232 and capacitor 234 in parallel is differentiated in the R. C. combination comprising capacitor 240 and resistor 242 to produce a signal having a wave form such as that shown at 244, the signal having a relatively large negative peak and a small positive peak. The signal together with the negative bias voltage supplied by battery 246 is impressed upon the control grid of vacuum tube 246, the grid circuit of which includes resistor 242 connected in series with battery 246 to ground. This battery furnishes sufficient bias to cut off this tube in the absence of applied signals. The cathode of this tube is grounded and the plate circuit includes load resistor 250 connected between the plate and the positive terminal of battery 252, the negative terminal of which is connected to ground.

The duration of the transmitting period is determined by the capacitance of capacitor 248, the greater this capacitance, the longer the transmitting period. Referring to the wave forms shown in Fig. 2 it will be recognized that for each negative saw-tooth pulse of the wave shown at 236 a dierentiated signal, such as that shown at 244, will be produced. The entire wave form 244 occurs during the period of each saw-tooth 236 when the operation of the counter is interrupted. The small positive pulse of wave 244 is employed to enable the transmitting equipment during some portion of this interval, the exact shape of the pulse being dependent upon the value of capacitor 240.

The positive pulse applied to the grid of tube 248 appears as an amplified negative pulse across load resistor 250 and this amplified pulse is applied through capacitor 253 to amplifier tube 254, the wave form of the voltage applied thereto being substantially as shown at 256. The negative pulse, however, has no effect on the output of tube 248 which is normally biased at cut-off. The grid of amplifier 254 is connected through resistor 258 to the negative terminal of bias battery 260, the positive terminal of which is grounded while the cathode is connected to ground. The plate of the tube is connected through resistor 262 to the positive terminal of battery 264, the negative terminal of which is grounded. Amplifier 254 serves to reverse the phase of the impressed signal and increase its level to a suitable value without materially altering its wave form. The positive pulse produced in the plate circuit of triode 254 is applied through capacitor 266 to the control grids of power amplifier tubes |90, |82 and |84, these control grids being connected in parallel through resistor 268 to the negativeterminal of bias battery 210, the positive terminal of which is grounded. The positive pulse so applied to the respective control grids of the power amplifier tubes permits current to ow therethrough during a part of the time that the operation of the counter is interrupted, if positive potentials from the counting stages associated with the amplier tubes are applied to the screen grids. Thus radio frequency current can now in power amplifier tubes |90, |82 or |94 only when a positive pulse from the timer occurs simultaneously with the application of a positive potential from the counting stages with which they are respectively associated.

In the operation of the coder and transmitter shown by way of example in Fig. 2 the complex wave form to be transmitted is applied through terminal equipment to comparator I8 in which Aits instantaneous amplitude measured from ground with a superimposed bias suieient to make all amplitudes of one polarity is compared with a voltage proportional in amplitude to the counting sum appearing at the counter output terminals |40 |38, and |36. If these two amplitudes differ, a dilerence of potential is generated between comparator output leads |56 and |58 and this difference of potential is applied to control tubes |64 and |66 to permit application of the output of counter oscillator |60 to the input of the counting circuit.

At the same time the polarity of the difference in potential between leads |56 and |58 determines the direction in which the counting circuit operates. The phase relationships throughout the counter control circuits are such that the counter will count to increase its sum output if the instantaneous amplitude of the complex wave to be transmitted exceeds the amplitude of voltage proportional to the sum output generated by the counter and counts downwardly if the opposite condition obtains.

In the presence of an amplitude difference, impulses from oscillator |60 after passage through control tubes |64 and |66 are applied to switching tube |12 through which they pass to counter input terminal 12 unless transmission of a code group is in progress. In this latter event, switching tube |12 is cut off and operation of the counter is momentarily interrupted for the duration of the transmisson. At any time the group of voltages appearing at the several counter output terminals |40 |38, and |36, respectively, of counter stages 58 56, and 54, taken in the order given, form a binary code representation of the number of impulses delivered by oscillator |60 and thus of the instantaneous amplitude of the complex wave to be transmitted. These voltages are applied to the separate subchannels of the radio transmitter which is switched on and off by the timer which serves also to interrupt the operation of the counting circuit at predetermined instants and synchronizes the operation of the two units. In those subchannels of the transmitter in which the transmitting period coincides with the presence of an on signal voltage at the output of the associated counting stage radio frequency energy is transmitted.

The receiving equipment indicated generally at 28, 30 and 32 in Fig. l may be of any suitable type, for example, radio receiver 38 may be a conventional superheterodyne receiver capable of broad band response, the intermediate frequency output of which will include a different frequency component for each of the activated subchannels. The intermediate frequency output of this recelver is applied to decoder 30 which is arranged to derive therefrom a complex wave form reproducing that impressed upon the input of the transmitting equipment. One form of decoder suitable for this purpose is disclosed in Fig. 5 of United States Patent 2,272,012, issued February 3, 1942, onl an application led in the name of Alec Harley Reeves. Briefly, the decoder disclosed in this patent includes a group of filters for separating the several alternating current voltages present in the intermediate frequency output of the receiver and applying them to a counter having the same number of stages as the counter employed in the transmitter. (In the patent five counting stages are employed.) These voltages are rectiiied and utilized to condition the counter so that the sum indicated thereon is the same as that indicated at the output terminals of the transmitter. The rectified output of the receiver is also utilized to trigger on an impulse generating circuit which after a slight delay causes the counter to count until its full capacity is reached. During the counting period current is permitted to now through an amplier, the duration of this current ilow being proportional therefore to the difference between the total capacity of the counter and the sum indicated by the transmitted binary code group of voltages. This output is ltered to obtain the desired reproduction of the complex wave form applied to the transmitter. For further details concerning this receiver and decoder reference is made to the above-identified patent.

What is claimd is:

1. In a communication system for transmitting complex waves, a coder for expressing the instantaneous amplitude of a complex wave to be transmitted in n-digit binary code comprising a reversible binary counter of n stages each stage having an operated and an unoperated condition, output connections from each of said stages providing an output voltage change when the corresponding stage is in the operated condition, and means responsive to the amplitude of the complex wave to be transmitted to control the direction of counting of said counter for maintaining the n-digit binary code group formed by the voltages appearing at said output connections representative of said amplitude.

2. In a communication system for transmitting complex waves, a coder for expressing the amplitude of a complexl wave to be transmitted in binary code of n denominational orders comprising a binary counter of n stages each stage having an on and an oft condition, means for deriving from each on stage a voltage proportional to the binary code amplitude represented thereby, and means responsive to the sum of these voltages and to the amplitude of the complex wave to be transmitted for controlling the operation of the counter.

3. In a communication system for transmitting complex waves, a coder for generating an n-digit binary code representative of the amplitude of a complex wave to be transmitted comprising an n-stage binary counter each stage having an operated and an unoperated condition, means for deriving from each operated stage a Voltage proportional to the binary code amplitude represented thereby, and means for comparing the sum of said voltages with the amplitude of the complex wave to be transmitted and causing the counter to operate whenever said sum differs from the amplitude of the complex wave.

4. In a communication system for transmitting complex waves, an n-stage binary counter for expressing the amplitude of a complex wave in n-digit binary code each counter stage having operated and unoperated conditions and an output connection at which a voltage change occurs when that stage is operated, the voltages at the output connections forming an n-digit binary code group, means for generating a voltage proportional to the amplitude represented by said code group, and means responsive to the difference between the amplitude of the complex wave 20 to be transmitted and the amplitude of said generated voltage for causing the counter to operate to equalize said amplitudes.

5. In a communication system for transmitting complex waves, a coder for expressing the amplitude of a complex wave in n-digit binary code comprising a reversible n-stage binary counter each stage having operated and unoperated conditions and an output connection at which a voltage change occurs when that stage is operated, the voltages at the output connections forming an n-digit binary code group, means for generating a voltage proportional to the amplitude represented by said code group, and means responsive to the diierence between the amplitude of the complex Wave to be transmitted and the amplitude of said generated voltage for causing the counter to operate in the proper direction to equalize said amplitudes.

6. In a communication system for transmitting complex waves, a counter for expressing the amplitude of a complex wave in n-digit binary code comprising an n-stage binary impulse counting circuit, an output connection for each stage, means for generating an output voltage for each operated stage proportional to the binary code amplitude represented thereby, a pulse generator for operating said counter, and control means for said pulse generator responsive to the sum of said output voltages and to said complex wave rendering it effective to operate the counter whenever the amplitude represented in binary code by the counter output voltages differs from the amplitude of the complex wave to be transmitted.

7. In a communication system for transmitting complex waves, a coder comprising an n-stage binary impulse counting circuit, an output connection for each stage, means for generating an output voltage proportional to the amplitude increment represented by each operated stage, said voltages forming a binary code group, a pulse generator for operating said counter, control means for said pulse generator responsive to the sum of the output voltages of said operated stages and to said complex wave for rendering said pulse generator eective to operate the counter whenever the amplitude represented in binary code by the counter output voltages differs from the amplitude of the complex wave to be transmitted, separate subchannels fortransmitting said counter output voltages to a receiving station, and means for disconnecting the pulse generator from the counter circuit at predetermined times and transmitting said output voltages over said subchannels.

8. The method of providing code signals representative of complex signals to be transmitted comprising continuously producing code groups in response to a complex signal to be transmitted, regenerating from said continuous code groups a representation of the signal to be transmitted, comparing said regenerated signal with the original complex signal, and correcting the generated code signals in accordance with the result of said comparison.

9. In a communication system for transmitting message waves, a coder for expressing the instantaneous amplitude of a message wave in binary code comprising means responsive to said wave for producing binary code groups of pulses, means for deriving from each of such code groups a voltage proportional to the amplitude represented thereby, means for comparing said derived voltage with the instantaneous amplitude of said message wave to obtain a control quantity and means responsive to said control quantity for correcting the code group output of said code group producing means.

10. An electric pulse code modulator for the simple addition binary code comprising a periodically operating counting device arranged to build up a potential wave the amplitude of which changes progressively in discrete steps, said counting device comprising a chain of binary counting stages each operated by the previous one, and means for applying to the first stage a train of regularly repeated trigger pulses, means for comparing the potential wave with a signal wave potential, means for stopping the operation of the counting device when the diference of the two wave potentials has reached a specified value, and means for deriving from each counting stage a potential representing one binary digit of the total number of successive binary digits characterizing the signal wave amplitude at ,substantially the given instant when the diference of the two wave potentials has reached said specified value.

11. A modulator according to claim 10 for a code of m units, comprising 7n binary counting stages each of which consists of a two-condition device and adapted to produce when operated an output-voltage proportional to .2f-1 where r is the number of the stage in the series and takes all values from 1 to m, and means for adding together the output voltages of all operated stages.

RAYMOND A. HEISING.

References cited in the me of this patent or the original patent UNITED STATES PATENTS 

